This invention relates to a method of improving the quality of x-ray images by adjusting the x-ray imaging system operating parameters during data acquisition.
Real industrial parts have rather complex shapes. This causes difficulty in choosing one set of x-ray inspection parameters which is appropriate for the entire part.
The x-ray absorption in a material of linear attenuation coefficient .alpha. and thickness L is given by EQU N.sub.1 =N.sub.o exp(-.alpha.L) (1)
where N.sub.o is the number of x-rays incident on the part and N.sub.1 is the number which penetrate it. If photon statistics are the dominant noise source, as they are in most cases for a well designed system, the signal to noise ratio in a digital radiography image is given by ##EQU1##
Based on this expression, it is simple to show that the optimum signal to noise ratio for inspection is achieved when .alpha.L=2. The x-ray attenuation coefficient .alpha. depends on x-ray energy, so it can be varied to improve the inspection signal to noise ratio. The counting noise in the image is proportional to the square root of the number of x-rays detected, so signal to noise at a particular value of .alpha.L can be improved by increasing the number of x-rays available for the measurement. This can be achieved either by increasing the x-ray tube current, i, or the measurement time .tau., or both since N.sub.1 is proportional to i.times..tau.. However, neither parameter can be increased without limit, since increased tube current causes increased heat transfer to the tube anode and eventual failure, and increased measurement time can cause failure to meet throughput requirements.
Since industrial parts can have widely varying thicknesses and can be made of materials with different x-ray absorptivities, it is clear that no single choice of inspection energy or integration time and tube current will be optimum for all sections of a complex part. Therefore, image quality is degraded over most of each image. Current x-ray inspection systems simply determine the best x-ray energy and data acquisition integration time for the part as a whole before the scan is started. For film inspection there is no real alternative to this approach. The entire 2-D image data set is acquired simultaneously and no adjustment is possible. However, often several images of the same part are taken at different energies or different integration times in order to provide useful information about different regions. This process is slow and difficult to interpret, however, since the images of different regions are separate pieces of film, and transition regions may not be imaged clearly in any shot.